Method of wet cleaning a surface, especially of a material of the silicon-germanium type

ABSTRACT

Method of wet cleaning a surface of at least one material chosen from silicon, silicon-germanium alloys, A(III)/B(V)-type semiconductors and epitaxially grown crystalline materials, such as germanium, in which method the following successive steps are carried out:
         a) the surface is brought into contact with an HF solution;   b) the surface is rinsed with acidified, deionized water, then a powerful oxidizing agent is added to the deionized water and the rinsing is continued;   c) optionally, step a) is repeated, once or twice, while optionally reducing the contacting time;   d) step b) is optionally repeated, once or twice; and   e) the surface is dried.       

     Process for fabricating an electronic, optical or optoelectronic device, such as a CMOS or MOSFET device, comprising at least one wet cleaning step using the said cleaning method.

This application claims priority under the provisions of 35 U.S.C. §119to French Application No. 03 51239, filed on Dec. 31, 2003 which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method of wet cleaning a surface, inparticular a surface of a material of the silicon-germanium type, andespecially of an Si_(1-x)Ge_(x) material where x is greater than 0 andless than 1, x preferably lying in the range from 0.1 to 0.9, morepreferably from 0.1 to 0.7.

The technical field of the invention can in general be defined as thatof the cleaning and decontamination of surfaces, in particular ofsurfaces of materials used in microelectronics, optics or electronics,especially surfaces of materials of the silicon-germanium alloy type,such as polished and crystalline Si_(1-x)Ge_(x). The contaminants thatmay be on these surfaces may be of any type, namely particulatecontaminants, organic contaminants, mineral contaminants, metalliccontaminants, etc.

Specifically, the constant development in microelectronic integratedcircuits based on CMOS silicon has been achieved thanks to theimprovement in their performance and to the miniaturization of theirelementary components. However, owing to the ever increasing density ofinterconnections, silicon would seem, in the near future, to be a majorphysical limitation. The SIA (Semiconductor Industry Association)roadmap has therefore anticipated the use of new materials. One of thematerials exhibiting the greatest potential, with many integrationoptions, for architectures of higher performance is germanium in theform of silicon-germanium alloys.

The material SiGe is deposited by epitaxy on silicon substrates. Thinsilicon layers, called “silicon-under-tension” layers, may be depositedon these substrates. This silicon-under-tension on SiGe appears to behighly advantageous owing to the electronic properties that it conferson the elementary components, such as MOSFET (metal-oxide semiconductorfield-effect transistor) or CMOS-type integrated circuit components.

The strained silicon may be a buried layer or a surface layer, and itgreatly increases the electron mobility. Various processes have beendescribed for forming this strained silicon on SiGe, using in particularthe SOI (Silicon On Insulator) process.

Thus, several processes for fabricating these SiGe relaxed substrateshave been described, such as for example in document [1]. One of theirmain characteristics is the presence of surface cross-hatching. Suchcross-hatching stems from the presence of dislocations that form andpropagate in a thick graded layer (with a thickness of several microns)at the base of these substrates. They are necessary in order for them tobe almost completely relaxed, the purpose being to maintain the lowestpossible defect density on the surface. As a result, this structure has,after epitaxy, a high surface roughness (several nm), which increaseswith the percentage of Ge in the alloy. Chemical-mechanical polishing(CMP) as described in document [2] is then necessary, before transfer ofan SiGe layer or re-epitaxy of strained silicon, in order to eliminatethis roughness and practically all the surface cross-hatching, so as toobtain a final surface roughness of less than 1 nm.

An optimized wet cleaning method is therefore of paramount importanceafter the CMP (chemical-mechanical polishing). It must be effective, soas to remove the contamination introduced by the polishing, composedespecially of microparticles and nanoparticles, organic contaminationand metallic contamination, without thereby degrading the initialsurface morphology. This point is particularly important as regardsrelaxed SiGe substrates, which exhibit different chemical propertiestowards certain cleaning solutions commonly employed inmicroelectronics.

Thus, an SC1 (Standard Cleaning 1) solution, which constitutes part ofthe RCA (Radio Corporation of America) cleaning procedure, the mostwidely used at the present time on silicon and other materials, causesvirulent etching of the SiGe surfaces. Even over short times and at lowtemperature, this treatment may create a surface microroughness, or evena major resurgence of surface cross-hatching, not compatible with theenvisaged electronic applications. This sensitivity to SC1 but also SC2(Standard Cleaning 2) solutions for germanium contents of greater than50% depends on the percentage of germanium concentration of the alloyand on the temperature of the solution. The pronounced roughening effectof this type of solution on the surface of Si_(1-x)Ge_(x) substrates(where x=0.3 to 0.7) has been recently reported in document [3]. The useof such chemistries is here and now unacceptable for technologicalintegration of SiGe materials with a high Ge content (≧30%).

A wet cleaning method aimed at overcoming the drawbacks of theconventional RCA cleaning procedure has been described in the literaturein the case of silicon substrates. This is what is called the DDC(Diluted Dynamic Clean) method that relies fundamentally on the use ofsolutions of dilute chemicals, at room temperature, and combines twochemical baths prepared in quartz tanks, one of these baths, dedicatedto the injection of chemicals, being recirculated and filtered.

This method is described in particular in documents [4] and [5] relatingthe pre-gate cleaning (i.e. gate oxide prediffusion cleaning) and isillustrated in FIG. 1. It employs a bath (1) in a rinsing tank (2), withan overflow (3), in which small quantities of reactants, such ashydrochloric acid or gaseous ozone, are injected together or alternately(at 4) into a stream of deionized water (5) that feeds the tank (6).

In FIG. 1, it should be noted that the reactants, such as HCl or O₃, areinjected (at 7) into the deionized water feed line (5) of the overflowtank (2), downstream of a static mixer (8) which is used to make themixture of the deionized water and the reactants homogeneous thanks tothe recirculation of the fluids as accomplished by the said static mixer(8).

The excess chemical bath of the tank is generally received by theoverflow (3) and is discharged (9). Such a tank makes it possible forthe cleaning method and the associated rinsing to be carried outalternately in the same bath. As a result, the space normally occupiedby the rinsing bath may be eliminated. If the quantity of chemicals isnegligible in the case of cleaning using dilute solutions, the overflowmay be discharged directly in such a way that the substrates, forexample the cleaned wafers, are always in contact with fresh reactants.

Another tank (10) contains a bath (11) consisting of a dilute solution,generally a 1% solution, of hydrofluoric acid in deionized water. Thisother tank comprises, so as to limit fluorinated discharges, arecirculation loop generally provided with a pump (12), a filter (13),an oxygen desorption device (14) in order to continuously eliminate thedissolved gases and a chemical purifier (15) in order to eliminate thenoble metals. The oxygen collected is discharged via the line (16).

A wet cleaning sequence tailored to gate oxide prediffusion cleaningwith silicon substrates therefore comprises the following successivesteps:

-   -   treatment with deionized water containing 3 ppm O₃ [4] or 20 ppm        O₃ [5], for 5 minutes in order to eliminate the noble metals and        the organic compounds;    -   treatment with a 1% HF solution, acidified by 1% of HCl, for        more than 0.5 minutes (overetch treatment) in order to eliminate        the sacrificial oxide and the metals;    -   rinsing for 5 minutes with deionized water (without any other        additive);    -   treatment with deionized water containing 3 ppm O₃ for 10        minutes and then with the above-described HF/HCl solution for 1        minute, so as to eliminate the particles;    -   rinsing for 5 minutes with deionized water acidified by HCl; and    -   treatment with ozonized water containing 3 ppm O₃ and acidified        with 0.01% HCl, in order to carry out a final passivation.

The drawbacks of the method of documents [4] and [5] applied to Sisubstrates are especially the following:

The etching of the silicon is very difficult to control, since siliconis very sensitive to etching by HF/HCl; the deoxidation time must bevery accurately adjusted so as to prevent any roughening of the silicon;excessive consumption of silicon results in problems in the subsequentdevice fabrication process steps, such as alignment and layer depositionproblems, and epilayer growth quality problems.

Moreover, the method of documents [4] and [5] is specifically designedfor and tailored to gate oxide prediffusion cleaning on siliconsubstrates and surfaces, and the steps of this method, the succession ofthese steps, their duration and the specific reactants that are employedduring each of the steps have been specifically optimized to eliminatecontamination, whether this be of organic, metallic, particulate orother origin, of such substrates, specifically silicon substrates, andthe effectiveness of this method has been demonstrated only in the caseof silicon surfaces.

There therefore exists a need for a method of wet cleaning surfaceswhich is of higher performance and greater effectiveness for cleaningsilicon substrates, and in particular there exists a need for a methodof cleaning surfaces made of silicon-germanium alloy, for exampleSi_(1-x)Ge_(x) surfaces (where x is greater than 0 and less than 1, forexample x ranging from 0.1 to 0.9) which, while ensuring very effectivecleaning and allowing elimination of all contaminations, whetherorganic, metallic, particulate or of other type, is at least equivalentto that of the conventional RCA cleaning procedure, but does not havethe drawbacks of the latter.

In other words, there exists a need for a method of wet cleaningsurfaces, particularly surfaces made of silicon-germanium alloys, whichis not only very effective and completely eliminates all thecontaminants, but does not cause roughening of the substrate and theresurgence of surface cross-hatching.

This method must also be reliable, easy to implement, comprise a limitednumber of steps, be of relatively short duration, use small quantitiesof easily available reactants and reduce the amounts discharged into theenvironment.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method of wet cleaningsurfaces, in particular surfaces made of silicon-germanium alloys,which, among other things, meets the needs and satisfies therequirements mentioned above.

The object of the invention is also to provide such a wet cleaningprocess that does not have the drawbacks, limitations, defects anddisadvantages of the cleaning methods of the prior art and solves theproblems of the prior art.

This object and yet others are achieved, in accordance with theinvention, by a method of wet cleaning a surface made of at least onematerial chosen from silicon, silicon-germanium alloys, A(III)/B(V)-typesemiconductors and epitaxially grown crystalline materials, such asgermanium, in which method the following successive steps are carriedout:

-   -   a) the surface is brought into contact with an HF solution with        an HF concentration of 0.2 to 2% by volume, preferably 1% by        volume, in deionized water (DIW), for a time of 10 minutes of        less, preferably 1 to 5 minutes, for example 4 minutes, the pH        of the said solution being maintained at a value of from 1 to 2,        preferably close to 1, throughout the duration of the        contacting;    -   b) the surface is rinsed with acidified, deionized water for a        time of 1 to 5 minutes, preferably 1 to 3 minutes, for example 3        minutes, then a powerful oxidizing agent is added to the        deionized water and the rinsing is continued for a time of 5 to        10 minutes, preferably 5 to 7 minutes, for example 7 minutes;        the pH being maintained at a value of 5 or less 5, preferably 3        to 5, throughout step b);    -   c) optionally, step a) is repeated, once or twice, while        optionally reducing the contacting time, which is then        preferably between 30 seconds and 2 minutes, for example 1        minute 30 seconds;    -   d) step b) is optionally repeated, once or twice; and    -   e) the surface is dried.

Advantageously, in step a) and/or step c), the pH is maintained byadding HCl to the HF solution, generally with a concentration of 0.5 to2% by volume, preferably 1% by volume, of the solution.

Advantageously, in step a) and/or step c), one or more complexing oroxidizing agents, such as HCl, are added to the solution, preferablywith a concentration of 0.5 to 2% by volume of the solution so as tomaintain the pH of the rinsing solution.

Advantageously, step a) and/or step c) is (are) carried out in a tankprovided with a recirculation loop preferably provided with a purifyingfilter and/or with a degasser and/or with a chemical purifier, whicheliminates the metallic and particulate contamination, in particular bynoble metals. More preferably, the above three items of equipment arepresent as such a configuration with three items of equipment allows thebest performance to be achieved.

Advantageously, in step b) and/or d), the pH is maintained by adding HClto the deionized water so as generally to obtain a concentration of 0.01to 1% by volume.

Advantageously, in step b) and/or d), the oxidizing agent is gaseousozone, added to the deionized water so as to obtain a concentrationgenerally of 3 to 15 ppm, preferably 6 ppm.

Advantageously, in step b) and/or d), the rinsing carried out ismegasonic rinsing, the megasonic waves preferably being applied for atime of 5 minutes or longer, more preferably 10 minutes or longer, forexample 10 to 20 minutes.

Advantageously, the drying of step c) is carried out by bringing thesurface into contact with isopropanol (Isopropyl Alcohol or IPA) or bycentrifugation.

Advantageously, the entire method is carried out at room temperature,namely, in general, from 20 to 30° C., preferably 22 to 25° C., forexample 23° C.

Advantageously, and prior to step a), a precleaning step a₀) is carriedout by bringing the surface into contact with an aqueous solutioncontaining ozone generally with a concentration of 6 ppm or more,preferably 6 to 20 ppm, for example 20 ppm, or with an H₂SO₄ solution towhich H₂O₂ has been added, namely in general 0.5 vol % or less of H₂O₂has been added, i.e. generally in proportions of 100 cm³ of H₂O₂ inundiluted H₂SO₄. In the latter case, the duration of contacting isadvantageously from 5 to 10 minutes, for example 5 minutes.

Preferably, the surface is a crystalline and/or polished surface.

The method according to the invention has a specific series of specificsteps, each defined by specific operating conditions such as the pH, thetemperature, the duration and essentially the nature and concentrationof the reactant(s) employed.

Such a method has never been disclosed or suggested in the prior art.

The method of the invention is distinguished from the methods of theprior art which make use of dilute reactants, such as those describedespecially in documents [4] and [5], in particular by the fact that, instep b), the pH is maintained at a value of less than 5, for example bythe addition of HCl. This means that, compared with the methods of theprior art, an additional acid rinsing operation is carried out. Thismodification of the methods of the prior art, such as those of documents[4] and [5], fundamentally improves the efficiency of the cleaning ofthese methods on silicon substrates, but also, surprisingly, on othersubstrates.

In other words, the method according to the invention makes it possible,whatever the type of cleaned surface—Si or other surface, and ingeneral, for a non-roughening and very effective cleaning operation tobe carried out, with a low consumption of reactants.

The improvements of the method according to the invention over themethods of the prior art, such as those of documents [4] and [5] areobtained for Si substrates but also for other substrates, such as inparticular SiGe substrates.

For example, the removal of particulate and metallic contamination isimproved on Si substrates and is achieved in an extremely easy manner,surprisingly, also on SiGe substrates. The same applies as regards lowsurface roughness and low HF concentration, which are improved whentreating Si substrates but which are also achieved, surprisingly and inan excellent manner, on SiGe substrates.

The method according to the invention may thus more precisely be definedas a DDC (Diluted Dynamic Clean)-type method which can be applied tosilicon-germanium alloy surfaces.

A similar method has already been described, as mentioned above [4] [5],but this was developed specifically for cleaning silicon surfaces andhas proved its feasibility and its effectiveness only for cleaning suchsilicon surfaces for which it does have, however, certain limitations asregards the subsequent microelectronic device fabrication steps.

Because of the fundamental differences that exist between, on the onehand, silicon surfaces and, on the other hand, silicon-germanium alloysurfaces, a cleaning method for a silicon surface can in no case betransposed directly to a silicon-germanium surface.

Furthermore, it is absolutely impossible to deduce from the fact that acleaning/rinsing solution and a method employing it give excellentresults for cleaning a silicon surface that this same solution will alsogive excellent results for cleaning an SiGe surface.

This is because the behaviour of a solution with respect to one surface(and vice versa) is completely unpredictable and its behaviour withrespect to a surface made of another material cannot be deducedtherefrom.

Astonishingly, it has been shown that the DDC-type method according tothe invention, although giving excellent results on silicon substrates,could also, surprisingly, be suitable for SiGe substrates and othersubstrates.

The method according to the invention overcomes the drawbacks of themethods of the prior art, meets the requirements mentioned above andsolves the problems of the methods of the prior art.

In fact, the method according to the invention, which may also bedefined for example as “ozone-based aqueous cleaning”, is an effectiveand non-roughening method carried out at room temperature and involvingan HF/ozone chemistry that has the same functions of removingcontamination, whether organic, metallic, particulate or othercontamination, as the conventional SC1 and SC2 type chemistriesconstituting the RCA cleaning procedure, but which, compared with thisprocedure, has many advantages such as a lower consumption of chemicals,a shorter cycle time, etc.

The method according to the invention makes it possible to carry out aneffective surface treatment that is non-roughening and is particularlysuitable for SiGe alloy surfaces, especially preferably for crystallineand/or polished Si_(1-x)Ge_(x) surfaces, where x is greater than 0 andless than 1, x preferably lying within the range from 0.1 to 0.9, morepreferably from 0.1 to 0.7 and even better still from 0.2 to 0.5.

The method according to the invention is particularly applicable toSi_(1-x)Ge_(x) surfaces with a high percentage of Ge, namely 20 to 70%germanium.

The method according to the invention has a high efficiency, possibly upto 70% to 90% in the case of all values of x, and it may be carried outwithout any particular modification whatever the germanium content ofthe alloy. In no case does the method according to the invention involveany degradation of the alloy layer.

A cleaning method more particularly suitable for SiGe surfacessatisfying, for example, the formula Si_(1-x)Ge_(x), where x lies withinthe ranges indicated above, comprises the following successive steps:

-   -   a₀) the surface is brought into contact with an H₂SO₄ solution        to which 0.5 vol % or less of H₂O₂ has been added (generally        corresponding to approximately 100 cm³ of H₂O₂), for a time of        about 5 minutes;    -   a) the surface is brought into contact with an HF/HCl solution,        having respective concentrations of 0.2 vol % and 1 vol %, in        deionized water, for a time of about 4 minutes;    -   b) the surface is rinsed with a 0.01 vol % HCl solution in        deionized water for a time of about 3 minutes;    -   c) the surface is rinsed with a 0.01 vol % HCl solution in        deionized water, into which 6 ppm of O₃ (concentration) has been        injected, in the presence of megasonic waves generally,        preferably, for a time as mentioned above for such a step;    -   d) step a) is repeated, the contacting time being about 1 minute        30 seconds;    -   e) step b) is repeated;    -   f) step c) is repeated; and    -   g) the surface is dried.

The advantages afforded by the method of the invention are inter aliathe following:

-   -   the particulate removal efficiency (PRE) obtained after cleaning        by the method of the invention is very high, close to 90%, and        varies depending on the Ge content present in the alloys, the        PRE being greater than 95% for Si_(0.8)Ge_(0.2) and close to 85%        for Si_(0.5)Ge_(0.5). The residual defects that are not cleaned        are ascribed to epitaxial defects;    -   the consumption of SiGe material brought about by cleaning using        the method of the invention is less than and negligible compared        with that obtained on silicon. This consumption is, for example,        20 Å for an SiGe containing 20 to 50% Ge. In contrast, in an        RCA-type medium in dilute chemistry, the high SiGe/Si        selectivity, which for example is greater than or equal to 4, is        mainly dependent on the concentration of the solutions used,        resulting in all cases in more pronounced etching of the alloy        with increasing % of Ge;    -   the surface morphology obtained, as observed by AFM (Atomic        Force Microscopy), is smooth with an R_(rms) (root mean square        roughness of various points on the surface) of generally <0.5        nm, is flat, that is to say without surface cross-hatching, and        is identical to a polished surface, whatever the Ge content of        the alloy. This morphology differs completely from that obtained        by dilute RCA cleaning, especially as soon as the Ge content is        above 30%;    -   in this regard, the reader may refer to FIG. 3, showing an        Si_(0.5)Ge_(0.5) surface after RCA cleaning with R_(rms)=0.8 nm        and R_(max)=8.05 nm; and to FIG. 4, showing an Si_(0.5)Ge_(0.5)        surface after cleaning according to the invention, with        R_(rms)=0.44 nm and R_(max)=2.83 nm. The visual comparison of        these mircrographs and of the R_(rms) and R_(max) values clearly        demonstrate the superiority of the method of the invention; and    -   in addition, analysis of the metallic contamination reveals the        absence of metallic contaminants after cleaning, or a negligible        contamination, close to the detection limit using a mass        spectrometry decomposition technique (VPD-ICPMS or Vapour Phase        Decomposition Inductively-Coupled Plasma Mass Spectrometry),        that is to say a contamination of less than 10¹⁰ at/cm².

It should be noted that this cleaning may be carried out not only on Sior SiGe surfaces, but also on SOI (Silicon On Insulator) substrates (thesurface is therefore made of Si), SsGOI (Strained Silicon on SiGe onInsulator) substrates and on surfaces composed of A(III)/B(V)-typematerials, such as InP, GaAs, etc., and of other epitaxially growncrystalline materials, for example germanium, by adapting the durationof the method and the HCl and HF concentrations.

Of course, the surface may be a composite surface comprising several ofthe abovementioned materials.

In conclusion, according to the invention the cleaning may be carriedout under dilute (HF) chemistry conditions without impairing theefficiency of the removal of the particulate and metallic contamination.

The invention also relates to a process for fabricating electronic,optical or optoelectronic devices, comprising at least one step of wetcleaning a surface of a material chosen from silicon, silicon-germaniumalloys and A(III)/B(V)-type semiconductors using the cleaning method asdescribed above.

For example, the said device may be chosen from CMOS (ComplementaryMetal-Oxide Semiconductor) devices and devices such as MOSFET(Metal-Oxide Semiconductor Field-Effect Transistor).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail in the followingdescription, given with reference to the appended drawings in which:

FIG. 1 is a schematic sectional view of the tanks used for implementinga DDC method and the method according to the invention;

FIG. 2 is a diagram showing the principle of how the cleaning methodaccording to the invention is integrated into a method for producingmicroelectronic structures;

FIG. 3 is a micrograph of an Si_(0.5)Ge_(0.5) surface observed by AFM(Atomic Force Microscopy) after cleaning of the dilute RCA type (Example3); and

FIG. 4 is a micrograph of an Si_(0.5)Ge_(0.5) surface observed by AFMafter cleaning by the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic vertical sectional view that describes the systemof two tanks, which are used to carry out a DDC (Diluted DynamicClean)-type method. Each of the two tanks, which are generally made ofquartz, contains a chemical bath. This tank system generally forms partof a robotized machine with automatic control of the baths as regardstheir preparation, their monitoring and their generation. This systemhas already been detailed above and is described in detail especially indocuments [4] and [5] to which the reader may refer. This system andthis machine may also be used for implementing the method of theinvention, by making a few adaptations that can be easily carried out bythe man skilled in the art.

The method according to the invention is a method of cleaning a surface.The term “cleaning” is understood in general according to the inventionto mean the removal, from the said surface, of undesirable contaminantsand pollutants on the surface. The method according to the inventionmakes it possible to remove any contaminant or pollutant, whatever itsnature. The contaminants and pollutants that are found on the surfacescleaned by the method according to the invention may be organic, mineralor metallic contaminants and pollutants and they may be present inparticulate or other form.

Among organic pollutants, mention may be made of hydrocarbons.

Among mineral pollutants, mention may be made, for example, of salts,such as nitrates and sodium chloride.

Even particulate contamination is removed very effectively by the methodof the invention.

The pollutants and contaminants are, for example, pollutants andcontaminants resulting from a prior treatment step carried out on thesurface, for example a chemical-mechanical polishing step.

The cleaning method according to the invention mainly comprises twocleaning steps which are, preferably, repeated once so that the cleaningmethod according to the invention generally comprises principally fourcleaning steps resulting from the repetition of a sequence of two stepscarried out in two chemical baths.

A first chemical bath (11), for deoxidation, is for example in the tank(10), which has a deionized water recirculation loop with a pump (12)illustrated in FIG. 1. This bath (11) employs a chemistry based ondilute HF, for example 1% HF, in deionized water; a second bath (1) forreoxidation is in the tank or reactor (2) having an overflow providedwith a static mixer (8) illustrated in FIG. 1. This bath employs achemistry essentially based on an acid, such as HCl, diluted indeionized water, with the optional addition of a strong oxidizing agent,such as ozone, which rinses the substrates under oxidizing conditions.

More precisely, the first step (step a)) of the method according to theinvention consists of deoxidation of the surface.

The purpose of this step is to remove the chemical or native oxide onthe surface by what is called an “underetching” mechanism.

The 49% hydrofluoric acid concentration (which is standard inmicroelectronics) used must be such that it allows complete deoxidationof the surface. For example, for effectively deoxidizing SiGe surfaces,without roughening the SiGe surface, an HF concentration of between 0.2%and 2% by volume may preferably be used.

More preferably, the HF concentration is 1% by volume.

The roughening of the surface, or the tendency to form pitting, due tothe oxidation of the silicon surface in the presence of noble metals, inan HF medium, must be avoided. To do this, oxidizing or complexingagents, such as HCl, may be added. In the case of SiGe surfaces, acombination of HF and HCl gases, with an HCl concentration preferablybetween 0.5 and 2% by volume, more preferably with a concentration of 1%by volume, makes it possible to maintain a pH close to 1 and thispromotes better removal of the metallic contamination.

In this first, etching step, the deoxidation time applied must generallybe sufficient to remove the chemical or native oxide present initiallyon the surface. In the case of an SiGe material, the deoxidation time isfor example less than 5 minutes so as to avoid roughening the surface byprolonged periods in which the substrate is in contact with HF.

In general, the duration of the contacting depends in particular on thematerial and on the thickness of the material that it is desired toremove.

The man skilled in the art will adapt this time according to the HFconcentration. For example, the usual time applied on SiGe is around 4minutes.

The etching rate will be determined experimentally by a person skilledin the art through successive trials.

To avoid any contamination of the bath by metals, such as Fe and Cu, andnoble metals such as Ag, a purification filter or “chemical purifier”(15)—that is to say a device containing a resin that complexes the metalions—may preferably be installed in the recirculation loop of the tankor reactor (10) in this step a) of the method according to theinvention.

Advantageously, the cleanliness of the baths is achieved by usinghigh-purity products, i.e. ULSI (Ultra Large Scale Integration) andSULSI (Super Ultra Large Scale Integration) products, namely productswhose content of particles, metals, phosphates, nitrates and chloridesis of the order of a few ppb, the main constituent of which is deionizedwater (DIW). The device is generally supplemented with continuousrecirculation through 0.1 μm filters (13).

This deoxidation step a) is also responsible for lifting off particles,due to the suppression of the oxide, which mechanism comes into play inthe removal of the particulate contamination.

In this step a) of the method according to the invention, the use of adilute chemistry employing HF baths has the advantage of reducing theconsumption of chemicals and the hazardous handling of chemicals.

The second step b) of the method of the invention consists in reformingor regenerating the oxide that has been removed during step a). It takesplace in deionized rinsing water from which particles have generallybeen filtered, and it involves surface reoxidation.

An acid pH, namely in general less than 5, must be maintained throughoutthe sequence in order to promote removal of the particulatecontamination and to maintain electrostatic repulsion conditions. Inparticular in the case of SiGe surface cleaning, this may entail a smallquantity of HCl acid, for example between 0.01% and 1%, being injectedinto the deionized water during the first few minutes of rinsing,without adding any other chemical. The acidification may be carried outby other acids, such as nitric acid, acetic acid or the like, ormixtures of acids.

The oxidation of the surface is carried out at room temperature thanksto the addition into the DIW of a powerful oxidizing agent. For example,if ozone is used as oxidizing agent for oxidizing SiGe surfaces, asufficient concentration must be applied so as to rapidly generate asufficient oxide thickness. The ozone concentration in the deionizedwater is generally 3 ppm to 15 ppm, preferably from 6 ppm to 15 ppm. Toincrease the effectiveness of particulate removal, acoustic activity,such as megasonic waves at a frequency for example of 1 to 1.8 MHz, maybe generated for a few minutes. Generally in our case, this is appliedfor a minimum duration of 5 minutes, preferably a minimum duration of 10minutes, for example 5 to 20 minutes.

As indicated above, the pH in the rinsing solution must be kept acid,namely generally at a value of less than 5.

The deoxidation/reoxidation sequence (first step a) and second step b))is generally repeated once or twice so as to ensure optimum removal ofthe particulate and metallic contamination.

Thus, when these steps are repeated once, the method according to theinvention will include a third step c) and a fourth step d).

The third step c) is identical to the first step a) in terms ofcomposition of chemicals. Only the contact time with HF may beshortened. It is generally between 30 seconds and 2 minutes in the caseof SiGe surfaces. By removing the oxide that has just been formed by thepreceding oxidation step, the underetching mechanism needed for removingthe particles is amplified.

The fourth step d) is identical to the second step b) in terms of timeand quantity of chemicals injected. Its sole purpose is to regeneratethe oxide removed during the third step, allowing final passivation ofthe surface.

Finally, after steps a) and b), or after step c) and d) if steps a) andb) are repeated, a final drying operation is carried out, for exampleusing isopropanol (isopropyl alcohol), or by centrifugation.

It should be noted that, before steps a) and b), which are optionallyrepeated, a chemical solution may be added upstream of the cleaningsequence. The purpose of this step a₀) is to reduce the initial organiccontamination before the essential deoxidation and reoxidation steps. Itprovides more a coarse decontamination or precleaning rather thanmeeting a requirement of use. This solution may be an aqueous solutioncontaining ozone, with a concentration of 6 ppm or less (6×10⁻³ g/l), ora CARO solution based on H₂SO₄ and H₂O₂ (with a concentration of 0.5 vol% or less).

FIG. 2 illustrates the manner whereby the cleaning method according tothe invention is integrated into a method for producing microelectronicstructures.

A layer of SiGe (21) of constant composition, for example containing 20to 50% Si, which is on a graded SiGe layer (having a Ge gradient) (22),itself on a (001) Si substrate (23), is subjected to a conventionalchemical-mechanical polishing operation (24). This polishing operationleaves the surface of the SiGe layer (21) with impurities (25) thatcomprise organic, mineral, metallic and particulate contaminants, thesecontaminants (25) all being removed by cleaning (26) using the method ofthe invention, after which a strained Si layer (27) is formed on theSiGe layer (21).

The method can then be continued via conventional steps that result inthe formation of structures such as strained Si-CMOS, strained Si-MOSFETand strained SOI-MOSFET structures.

The invention will now be described with reference to the followingexamples, given by way of illustration but implying no limitation.

EXAMPLE 1

In this example, the cleaning was carried out using the method accordingto the invention on a base substrate possessing the following structure:

-   -   <100> Si_(0.5)Ge_(0.5) relaxed substrate having a total        thickness of greater than 2 μm, with a graded layer of thickness        greater than 1 μm and a layer of constant composition with a        thickness close to 1 μm;    -   the said relaxed substrate was grown epitaxially on a lightly        p-doped (7 to 10 Ω·m)<100> Si substrate.

The cleaning according to the invention comprised the followingsuccessive steps:

-   -   “decontaminating” cleaning with an H₂SO₄ solution to which 100        cm³ of H₂O₂ had been added—this is step a₀) of the method        according to the invention;    -   automated cleaning, broken down into two successive main steps:    -   step 1: contacting of the substrate with an HF solution        containing 0.20% HF and 1% HCl in deionized water, for a time of        4 minutes, at ordinary ambient temperature (close to 20° C.).        This is step a) of the method according to the invention, which        allows the chemical native oxide present on the surface of the        substrate to be removed;    -   step 2: contacting of the substrate with a solution containing        0.01% HCl in deionized water for a period of 3 minutes followed        by injection of gaseous O₃, and continuation of the contacting        with the acidified solution containing 6 ppm O₃ for a time of 7        minutes, in the presence of 1.8 MHz megasonic waves. This is        step b) of the method of the invention, which allows a        protective chemical oxide with a thickness close to about 20 Å        to be reformed.

Steps 1 and 2 were repeated once, using the same operating conditions,apart from the duration of the second deoxidation (repetition of step1), which was now only 1 minute 30 seconds instead of 4 minutes.

Repeating these steps ensures optimum removal of the particulatecontamination resulting from the chemical-mechanical polishing residues,i.e. close to 85% removal, which is only limited by the presence of manyepitaxial defects at the wafer edge.

After cleaning by the method of the invention, a final cleaningoperation was generally carried out using a 0.2% HF solution, called “HFlast” (this cleaning does not in general fall within the context of themethod of the invention—it is a complementary method), and then anannealing operation was carried out in a hydrogen atmosphere at 800° C.,for 2 minutes, at a pressure of 20 torr, followed by re-epitaxy in anRP-CVD (reduced-pressure chemical vapour deposition) chamber, which madeit possible, at 750° C., to regrow silicon with a thickness of 50 nmusing DCS (dichlorosilane (SiH₂Cl₂)+silane (SiH₄) chemistry.

EXAMPLE 2

In this example, the cleaning was carried out by the method of theinvention on a base substrate possessing the following structure:

-   -   transferred <100> Si_(0.8)Ge_(0.2) on insulator substrate (SGOI,        i.e. SiGe on insulator structure).

The cleaning according to the invention comprised the followingsuccessive steps:

-   -   decontaminating cleaning using a pure H₂SO₄ solution (96 vol %        H₂SO₄, standard in microelectronics), to which 100 cm³ of H₂O₂        had been added. This is step a₀) of the method according to the        invention;    -   automated cleaning, broken down into two successive main steps:    -   step 1: contacting of the substrate with an HF solution        containing 0.20% HF and 1% HCl in deionized water, for a time of        4 minutes, at ordinary ambient temperature (close to 20° C.).        This is step a) of the method according to the invention, which        allows the chemical or native oxide present on the surface of        the substrate to be removed;    -   step 2: contacting of the substrate with a solution containing        0.01% HCl in deionized water for a period of 3 minutes followed        by injection of gaseous O₃, and continuation of the contacting        with the acidified solution containing 6 ppm O₃ for a time of 7        minutes, in the presence of 1.8 MHz megasonic waves. This is        step b) of the method of the invention, which allows a        protective chemical oxide with a thickness close to about 20 Å        to be reformed.

Steps 1 and 2 are repeated once.

After cleaning by the method of the invention, a final HF cleaningoperation, called “HF last”, was carried out, and then an annealingoperation was carried out in a hydrogen atmosphere at 800° C., for 2minutes, at a pressure of 20 torr, followed by re-epitaxy in an RP-CVDchamber, which made it possible, at 750° C., to regrow 10 nm ofSi_(0.8)Ge_(0.2), 10 nm of Si and 30 nm of Si_(0.8)Ge_(0.2) (using DCSchemistry).

EXAMPLE 3 (COMPARATIVE EXAMPLE)

An Si_(0.5)Ge_(0.5) surface was cleaned by an RCA-type method underdilute conditions, at a temperature below 40° C.

The surface was firstly treated with an NH₄OH/H₂O₂/H₂O solution (withvolume ratios of 0.012/1/20) and then with an HCl/H₂O₂/H₂O solution(with volume ratios of 1/1/80).

The surface obtained after the cleaning was observed by AFM (AtomicForce Microscopy). FIG. 3 is a micrograph of a square having an area of20 μm by 20 μm taken during this observation. The treated surface had anR_(rms) of 0.8 nm and an R_(max) of 8.05 nm.

This FIG. 3 is to be compared with FIG. 4, which is a micrograph of asquare of area 20 μm by 20 μm taken during observation of a surface byAFM after a cleaning treatment using the DDC method of the invention.

The treated surface has an R_(rms) of 0.44 nm and an R_(max) of 2.83 nm,these being very much lower than the values in FIG. 3 (prior art).

Simple visual comparison of the micrographs in FIGS. 3 and 4 clearlyshows that a surface of much greater quality is obtained after cleaningusing the method of the invention.

REFERENCES

-   [1] J. M. Hartmann, B. Gallas, J. Zhang and J. J. Harris, Semicond.    Sci. Technol. 15 (2000), 370.-   [2] K. Sawano, K. Kawaguchi, T. Ueno, S. Koh, K. Nakagawa and Y.    Shiroki, Mat. Sci. Eng. B89 (2002), 406.-   [3] K. Sawano, K. Kawaguchi, S. Koh, Y. Hirose, T. Hattori, K.    Nakagawa and Y. Shiraki, J. Electrochem. Soc. 150 (2003), G376.-   [4] F. Tardif, T. Lardin, P. Boelen, R. Novak and I. Kashkouch,    Proceedings of the 3rd International Symposium, UCPSS 1996, 175.-   [5] F. Tardif, T. Lardin, A. Danel, P. Boelen, C. Cowache, I.    Kashkoush and R. Novak, Proceedings of the 4th International    Symposium, UCPSSS 1998, 19.

1. A method of wet cleaning a surface in which the following successivesteps are carried out: a_(o)) the surface is brought into contact withan H₂SO₄ solution to which 0.5 vol % or less of H₂O₂ has been added, fora time of about 5 minutes; a) the surface is brought into contact withan HF/HCL solution, having respective concentrations of 0.2 vol % and 1vol %, in deionized water, for a time of about 4 minutes; b) the surfaceis rinsed with a 0.01 wt % HCL solution in deionized water for a time ofabout 3 minutes; c) the surface is rinsed with a 0.01 vol % HCL solutionin deionized water, into which 6 ppm of O₃ has been injected, in thepresence of megasonic waves; d) step a) is repeated, the contacting timebeing about 1 minute 30 seconds; e) step b) is repeated; f) step c) isrepeated; and g) the surface is dried, wherein the surface is made of asilicon-germanium alloy of formula Si_(1-x)Ge_(x) where x is greaterthan 0 and less than 1, x preferably lying within the range of 0.1 to0.9, more preferably from 0.1 to 0.7 and better still from 0.2 to 0.5.2. The method according to claim 1, wherein the surface is a surface ofan electronic, optical or optoelectronic device.
 3. The method accordingto claim 2, wherein the device is selected from CMOS (ComplementaryMetal-Oxide Semiconductor) devices and MOSFET (Metal-Oxide SemiconductorField-Effect Transistor) devices.